Electronic devices with integrated circuit chips or semiconductor dies encapsulated in plastic packaging have been employed for some years. In the present description, an electronic device refers to a packaged semiconductor device incorporating one or more integrated circuit chips. Typically, an epoxy resin is molded around the semiconductor chip, a central portion of a lead frame and bonding wires or other connections between contact pads on the chip to inner lead fingers on the lead frame. It is sometimes necessary to decapsulate such a plastic-encapsulated package at least in part to allow for inspection, test and repair of the chip or the wire bonds to the chip, or the inner lead fingers, after the epoxy covering these elements is safely and effectively removed. In general, concentrated acids such as sulfuric and nitric acids or other solvents for the resin have been used in a decapsulation system for removing plastic material to expose the encapsulated chip or package elements.
Epoxy molding compounds have been used extensively as an encapsulation material for semiconductor devices. Epoxy molding compounds are at times classified as trade secrets by device manufacturers as the epoxy molding compound supports the reliability of the semiconductor die and ensures performance in harsh operational environments. The standard composition of a semiconductor epoxy molding compound can include an epoxy resin, a hardener, an accelerator, a filler, coupling compound, flame retardant compound, a coloring agent, and a releasing compound. The main content of the epoxy molding compound is the silica fillers which are important as the fillers determine the reliability of the semiconductor device, including characteristics such as the moisture tolerance and temperature conductivity during operation. The fillers can constitute up to 75% to 90% of the total weight of the epoxy molding compound used for the encapsulation of a semiconductor device.
Although an electronic device may be packaged in a variety of ways using different types of encapsulation materials, the present description is concerned with electronic devices that are encapsulated in plastic packaging or other types of resinous materials. That is, the electronic device includes plastic encapsulation covering at least the chip(s), a central portion of a lead frame and bond wires connecting the chip(s) to the lead frame. Bond wires are made of various conductive materials, with copper (Cu) bond wires and gold (Au) bond wires being most commonly used. The electronic device also includes leads or lead fingers projecting out of the plastic encapsulation where the leads or lead fingers are used to form electrical connections from the outside world to the integrated circuit chip(s) encapsulated therein.
Besides gold and copper bond wires, palladium coated Cu wire, with about 0.08 μm layer of palladium over copper has also been used. Furthermore, silver alloy (Ag Alloy) has also been used as an alternative to gold bond wires as alloying silver has been useful in eliminating the silver migration phenomenon. Ag alloy wires have low Young's modulus, hardness properties, low cost compared to Au and PdCu bond wires and have bonding hardness similar to Au wire bonding which lead device manufacturers to make use of Ag alloy bond wires.
As described above, concentrated acids such as sulfuric and nitric acids or other solvents for the resin have been used in a decapsulation system for removing the epoxy molding compound material to expose the encapsulated chip or package elements. The conventional decapsulation process is an entirely wet etch process. The wet-etch decapsulation process has its limitations when semiconductor devices employ Cu and Ag based bond wires. Copper and silver alloy bond wires tend to suffer from etch damage when subjected to the conventional web-etch based decapsulation process.
Plasma etching is a dry etch process employed in semiconductor fabrication processes to etch or remove materials from a semiconductor chip, such as silicon, silicon oxide, silicon nitride or metallization materials. Plasma etching is carried out in a vacuum chamber where the plasma source is often excited by a radio frequency power source. Plasma etching is accomplished by ion bombardment and chemical reaction. Reactive-ion etching (RIE) is a plasma etch process commonly employed in semiconductor failure analysis for deprocessing a completed semiconductor die. RIE is typically carried out at a high frequency (e.g. 13.56 MHz). Attempts have been made to use RIE plasma etch system for plastic package decapsulation. However, using RIE plasma etch system for decapsulation is not practical as the decapsulation process could take several hours. During such a decapsulation process, the semiconductor device is subjected to high temperature which may have the undesired consequence of healing bake recoverable failures on the semiconductor die.